This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The faithful inheritance of prokaryotic genetic material requires the directed movement and positioning of chromosomes and plasmids to daughter cells at cell division. This process, called partition or segregation, is mediated by functionally homologous par systems comprised of a cis-acting centromere-like DNA site(s) and two proteins, ParA and ParB. The Escherichia coli P1 plasmid partition apparatus has served as a paradigm for partition. P1 ParA is a 44 kDa Walker type ATPase that drives plasmid separation at the final step of partition. P1 ParB is a 38 kDa DNA-binding protein that mediates the initial steps in segregation;partition complex formation and pairing. In partition complex formation, ParB and the E. coli protein, integration host factor (IHF), bind cooperatively to the ~74 bp parS centromere-like site, which contains multiple A- and B-Boxes, to form the partition complex. Although the P1 Par system has been biochemically well characterized, a detailed mechanistic understanding of partition is lacking due, in large part, to the dearth of structural information on partition proteins and their complexes. We attempt to obtain three dimensional structures of the complexes of these components by solution x-ray scattering to understand molecular mechanism of partition. We have constructed a structural model of the truncated ParB-IHF-parS partition complex, based on the available high resolution structures of its components. Our goals are to first verify that model by solution x-ray scattering, and further build three-dimensional structural models of larger partition complexes involving full length ParB and different types of parS, eventually the pre-segregation complex ParB-IHF-ParA(ATP)-parS.